Manual external defibrillator
The units are used in conjunction with or more often have inbuilt electrocardiogram readers, which the clinician uses to diagnose a cardiac condition most often fibrillation or tachycardia although there are some other rhythms which can be treated by different shocks. The clinician will then decide what charge to use which is measured in joules, based on their prior knowledge and experience, and will deliver the shock through paddles or pads on the patient's chest. As they require detailed medical knowledge, these units are generally only found in hospitals and on some ambulances. In the United States, paramedics are trained to recognize lethal arrhythmias and deliver appropriate electrical therapy with a manual defibrillator, when appropriate.
Manual internal defibrillator
Manual internal defibrillator are virtually identical to the external version, except that the charge is delivered through internal paddles in direct contact with the heart. These are almost exclusively found in operating theatres, where the chest is likely to be open, or can be opened quickly by a surgeon.
Automated external defibrillator or AED
These simple to use units are based on computer technology which is designed to analyze the heart rhythm itself, and then advise whether a shock is required. They are designed to be used by lay persons, who require little training. They are usually limited in their interventions to delivering high joule shocks for VF and VT (ventricular tachycardia) rhythms, making them generally limiting for use by health professionals, who could diagnose and treat a wider range of problems with a manual or semi-automatic unit.
The automatic units also take time generally 10-20 seconds to diagnose the rhythm, where a professional could diagnose and treat the condition far quicker with a manual unit. These time intervals for analysis, which require stopping chest compressions, have been shown in a number of studies to have a significant negative effect on shock success. This effect led to the recent change in the AHA defibrillation guideline (calling for two minutes of CPR after each shock without analyzing the cardiac rhythm) and a statement in the ACLS Provider Manual that AEDs should not be used when manual defibrillators and trained operators are available.
Automated external defibrillators are generally either held by trained personnel who will attend incidents, or are public access units which can be found in places including corporate and government offices, shopping centres, airports, restaurants, casinos, hotels, sports stadiums, schools and universities, community centres, fitness centres and health clubs.The locating of a public access AED should take in to account where large groups of people gather, and the risk category associated with these people, to ascertain whether the risk of a sudden cardiac arrest incident is high. For example, a centre for teenage children is a particularly low risk category (as children very rarely enter heart rhythms such as VF(Ventricular Fibrillation or VT(Ventricular Tachycardia), being generally young and fit, and the most common cause of paediatric cardiac arrest is trauma - where the heart is more likely to enter asystole or PEA, where an AED is of no use), whereas a large office building with a high ratio of males over 50 is a very high risk environment.
In many areas, emergency services vehicles are likely to carry AEDs, with some Ambulances carrying an AED in addition to a manual unit. In addition, some police or fire service vehicles carry an AED for first responder use. Some areas have dedicated community first responders, who are volunteers tasking with keeping an AED and taking it to any victims in their area. It is also increasingly common to find AEDs on transport such as commercial airlines and cruise ships.
In order to make them highly visible, public access AEDs often are brightly coloured, and are mounted in protective cases near the entrance of a building. When these protective cases are opened, and the defibrillator removed, some will sound a buzzer to alert nearby staff to their removal but do not necessarily summon emergency services. All trained AED operators should also know to phone for an ambulance when sending for or using an AED, as the patient will be unconscious, which always requires ambulance attendance.
Semi-automated external defibrillators
These units are a compromise between a full manual unit and an automated unit. They are mostly used by pre-hospital care professionals such as paramedics and emergency medical technicians. These units have the automated capabilities of the AED but also feature an ECG display, and a manual override, where the clinician can make their own decision, either before or instead of the computer. Some of these units are also able to act as a pacemaker if the heart rate is too slow (bradycardia) and perform other functions which require a skilled operator.
Implantable Cardioverter-defibrillator or ICD
Also known as automatic internal cardiac defibrillator (AICD). These devices are implants, similar to pacemakers (and many can also perform the pacemaking function). They constantly monitor the patient's heart rhythm, and automatically administer shocks for various life threatening arrhythmias, according to the device's programming. Many modern devices can distinguish between ventricular fibrillation, ventricular tachycardia, and more benign arrhythmias like supraventricular tachycardia and atrial fibrillation. Some devices may attempt overdrive pacing prior to synchronised cardioversion. When the life threatening arrhythmia is ventricular fibrillation, the device is programmed to proceed immediately to an unsynchronized shock.
There are cases where the patient's ICD may fire constantly or inappropriately This is considered a medical emergency, as it depletes the device's battery life, causes significant discomfort and anxiety to the patient, and in some cases may actually trigger life threatening arrhythmias. Some emergency medical services personnel are now equipped with a ring magnet to place over the device, which effectively disables the shock function of the device while still allowing the pacemaker to function (if the device is so equipped). If the device is shocking frequently, but appropriately, EMS personnel may administer sedation.
Modeling Defibrillation
The efficacy of a cardiac defibrillator is highly dependent on the position of its electrodes. Most internal defibrillators are implanted in octogenarians, but a few children need the devices. Implanting defibrillators in kids is particularly difficult because children are small, will grow over time, and possess cardiac anatomy that differs from that of adults. Recently, researchers were able to create a software modeling system capable of mapping an individual’s thorax and determining the optimal position for an external or internal cardiac defibrillator.
With the help of pre-existing surgical planning applications, the software uses myocardial voltage gradients to predict the likelihood of successful defibrillation. According to the critical mass hypothesis, defibrillation is effective only if it produces a threshold voltage gradient in a large fraction of the myocardial mass. Usually, a gradient of three to five volts per centimeter is needed in 95 % of the heart. Voltage gradients of over 60 V/cm can damage tissue. The modeling software seeks to obtain safe voltage gradients above the defibrillation threshold.
Early simulations using the software suggest that small changes in electrode positioning can have large effects on defibrillation, and despite engineering hurdles that remain, the modeling system promises to help guide the placement of implanted defibrillators in children and adults.
Interface with the patient
The most well-known type of electrode is the traditional metal paddle with an insulated handle. This type must be held in place on the patient's skin while a shock or a series of shocks is delivered. Before the paddle is used, a gel must be applied to the patient's skin, in order to ensure a good connection and to minimize electrical resistance, also called chest impedance (despite the DC discharge). These are generally only found on the manual external units.
Another type of resuscitation electrode is designed as an adhesive pad. When a patient has been admitted due to heart problems, and the physician or nurse has determined that he or she is at risk of arrhythmia, they may apply adhesive electrodes to the patient in anticipation of any problems that may arise. These electrodes are left connected to a defibrillator. If defibrillation is required, the machine is charged, and the shock is delivered, without any need to apply any gel or to retrieve and place any paddles. These adhesive pads are found on most automated and semi-automated units.
Both solid- and wet-gel adhesive electrodes are available. Solid-gel electrodes are more convenient, because there is no need to clean the patient's skin after removing the electrodes. However, the use of solid-gel electrodes presents a higher risk of burns during defibrillation, since wet-gel electrodes more evenly conduct electricity into the body.
Adhesive electrodes are designed to be used not only for defibrillation, but also for non-invasive pacing and electrical cardioversion.
While the paddles on a monitor/defibrillator may be quicker than using the patches, adhesive patches are superior due to their ability to provide appropriate EKG tracing without the artifact visible from human interference with the paddles. Adhesive electrodes are also inherently safer than the paddles for the operator of the defibrillator to use, as they minimize the risk of the operator coming into physical (and thus electrical) contact with the patient as the shock is delivered, by allowing the operator to stand several feet away. Another inconvenience of the paddles is the requirement of around 25 lb of pressure to be applied while defibrillating.
Placement
Resuscitation electrodes are placed according to one of two schemes. The anterior-posterior scheme is the preferred scheme for long-term electrode placement. One electrode is placed over the left precordium (the lower part of the chest, in front of the heart). The other electrode is placed on the back, behind the heart in the region between the scapula. This placement is preferred because it is best for non-invasive pacing.
Picture Above: Anterio-apical placement of external defibrillator electrodes (When defibrillation is unsuccessful, anterio-posterior placement is also sometimes attempted)The anterior-apex scheme can be used when the anterior-posterior scheme is inconvenient or unnecessary. In this scheme, the anterior electrode is placed on the right, below the clavicle. The apex electrode is applied to the left side of the patient, just below and to the left of the pectoral muscle. This scheme works well for defibrillation and cardioversion, as well as for monitoring an ECG.
Source - http://www.wikipedia.org
No comments:
Post a Comment